Each spring in the Arctic, the flooding triggered by melting snow which washes vast amounts of carbon-rich soil from the land into the streams and ocean which is a process called slumping. This may not sound like much. Certainly it is localized erosion, but global warming? That's of particular interest to scientists studying global warming, because in those waters much of the carbon that's being released from melting permafrost is oxidized by bacteria into carbon dioxide, says Rose Cory, an environmental scientist at the University of North Carolina, Chapel Hill. Carbon from surface waters amounts to as much as 40% of the total carbon that ultimately gets transferred from the Arctic to the atmosphere, she says.

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Cory studies the process by which organic carbon, trapped in frozen permafrost soils in the Arctic for thousands of years, may be converted to carbon dioxide, a greenhouse gas, and the potential role of that chemical conversion in global warming.

"We know that increasing global temperatures will thaw these ancient arctic soils, leading to increased opportunities for this carbon to be released as carbon dioxide, with the potential to reinforce the warming effect of carbon dioxide at global scales," she said. "But we need to understand the chemistry and magnitude of these alterations in the arctic carbon cycle, and how these changes will influence the climate of our world in the next 50 to 100 years."

Relatively little research has been done on the transformation of organic carbon in Arctic waters into carbon dioxide. Instead, much of the work on permafrost degradation over the past few decades has focused on the widespread, slow melting of the icy soils from the top down.

As melting has progressed in the Arctic, large holes and landslides have popped up across the tundra. In permafrost, ice holds up the soil; when the ice melts, the land surface slumps, creating features known as thermokarst failures. A slump is a form of mass wasting that occurs when a coherent mass of loosely consolidated materials or rock layers moves a short distance down a slope. Movement is characterized by sliding along a concave-upward or planar surface. Causes of slumping include earthquake shocks, thorough wetting, freezing and thawing, undercutting, and loading of a slope.

To their surprise, the researchers found that the samples from the thermokarst sites had lower levels of colored dissolved organic matter than did reference sites, suggesting that the carbon in the deeper soils exposed by thermokarst failure is significantly different from the carbon draining from the topmost, active layer of the permafrost, the team reports in the Proceedings of the National Academy of Sciences. When the team determined what happened to that carbon under UV light, they found that the deeper carbon was also about 40% more susceptible to photochemical and biochemical degradation into CO2 than was carbon from the active layer.

"What it means is that the carbon coming out of these sites is more reactive" than the carbon draining from the active layer of permafrost, Cory says. "[So] there is potential that these tremendous stores of carbon in these soils can be a positive feedback for more warming."

Understanding the transformations of the carbon in Arctic waters—including the paper's finding that exposure to light accelerates the decomposition—will be key to understanding the effect on climate, says Vladimir Romanovsky, a geophysicist at the University of Alaska, Fairbanks, who was not involved in the new study.